Layered Catalyst To Improve Selectivity Or Activity Of Manganese Containing Vanadium-Free Mobile Catalyst

ABSTRACT

Low temperature activity and high temperature ammonia selectivity of a vanadium-free selective catalytic reduction catalyst are controlled with a mixed oxide support containing oxides of titanium and zirconium, and a plurality of alternating layers respectively formed of a metal compound and titanium oxide present on the surface of the mixed oxide support. The metal compound is selected from the group consisting of manganese oxide, iron oxide, cerium oxide, tin oxide, and mixtures thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 12/463,828, filedMay 11, 2009, the contents of which are expressly incorporated herein inits entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

1. Field of Invention

The invention relates generally to catalysts and methods of makingcatalysts and, more particularly, but not by way of limitation, tocatalysts and methods of making catalysts that are useful for purifyingexhaust gases and waste gases from combustion processes.

2. Background of the Invention

The high temperature combustion of fossil fuels or coal in the presenceof oxygen leads to the production of unwanted nitrogren oxides (NO_(x)).Significant research and commercial efforts have sought to prevent theproduction of these well-known pollutants, or to remove these materialsprior to their release into the air. Additionally, federal legislationhas imposed increasingly more stringent requirements to reduce theamount of nitrogen oxides released to the atmosphere.

Processes for the removal of NO_(x) formed in combustion exit gases arewell known in the art. The selective catalytic reduction (SCR) processis particularly effective. In this process, nitrogen oxides are reducedby ammonia (or another reducing agent such as unburned hydrocarbonspresent in the waste gas effluent) in the presence of a catalyst withthe formation of nitrogen. Effective SCR DeNO_(x) catalysts include avariety of mixed metal oxide catalysts, including vanadium oxidesupported on an anatase form of titanium dioxide (see, for example, U.S.Pat. No. 4,048,112) and titania and at least the oxide of molybdenum,tungsten, iron, vanadium, nickel, cobalt, copper, chromium or uranium(see, for example, U.S. Pat. No. 4,085,193).

A particularly effective catalyst for the selective catalytic reductionof NO_(x) is a metal oxide catalyst comprising titanium dioxide,divanadium pentoxide, and tungsten trioxide and/or molybdenum trioxide(U.S. Pat. No. 3,279,884). Also, U.S. Pat. Appl. Pub. No. 2006/0084569(projected U.S. Pat. No. 7,491,676) teaches a method of producing animproved catalyst made of titanium dioxide, vanadium oxide and asupported metal oxide, wherein the titania supported metal oxide has anisoelectric point of less than or equal to a pH of 3.75 prior todepositing the vanadium oxide.

Vanadium and tungsten oxides supported on titania have been standardcatalyst compositions for NO_(x) reduction since its discovery in the1970's. In fact, very few alternatives rival the catalytic performanceof vanadium and tungsten oxides supported on titania. Despite theperformance advantages of vanadium and tungsten oxides supported ontitania, it would be advantageous to replace tungsten and/or vanadiumwith alternative metal components due to the significant drawbacks withusing both tungsten and vanadium in SCR catalysts. First, tungstenshortages have led to increased costs associated with its use. Second,the potential toxicity of vanadium oxide has led to health concernsregarding its use in selective catalytic reduction DeNO_(x) catalystsfor mobile applications, as well as significant costs associated withdisposal of spent catalysts.

It is known in the art that iron-supported titanium dioxide is aneffective selective catalytic reduction DeNO_(x) catalyst (see, forexample, U.S. Pat. No. 4,085,193). However, the limitations to usingiron as an alternative are its lower relative activity and, bycomparison, a high rate of oxidation of sulfur dioxide to sulfurtrioxide (see, for example, Canadian Pat. No. 2,496,861). Anotheralternative being proposed is transition metals supported on betazeolites (see for example, U.S Pat. Appl. Pub. No. 2006/0029535). Thelimitation of this technology is the high cost of zeolite catalysts,which can be a factor of 10 greater than comparable titania supportedcatalysts.

For implementation of lean burn engine technologies, the SCR DeNO_(x)catalyst used must have the capability of achieving very high reductionof NO_(x) over a broad range of temperatures, for example at least therange of 250° C. to 450° C. Most catalysts for lean burn applicationsexhibit satisfactory performance over only a fairly narrow temperaturerange; therefore, suitable catalysts are the focus of considerableresearch. Manganese oxide-based catalysts have been suggested for use aslow temperature SCR DeNO_(x) catalysts, as have similar iron, cerium,copper, tin oxide-base catalysts. However, the manganese oxide-basedcatalysts are limited at higher temperatures due to low ammoniaselectivity. Another disadvantage when using manganese is its highselectivity for N₂O formation, which contributes to ozone formation andacts as a greenhouse gas.

There remains a need for catalysts that exhibit improved performance forselective catalytic reduction of NO_(x) in the presence of ammonia overat least a temperature range of 250° C. to 450° C. To this end, it isdesirable to improve the ammonia selectivity of manganese, iron, ceriumand/or tin containing SCR DeNO_(x) catalysts at temperatures of 450° C.and above, while providing improved conversion activity at temperaturesof 250° C. and below, as well.

SUMMARY OF THE INVENTION

The present invention is directed to a catalyst composition having amixed oxide support containing oxides of titanium and zirconium. Aplurality of alternating layers respectively formed of a metal compoundand titanium oxide are present on the surface of the mixed oxidesupport. The metal compound is an oxide of manganese, iron, cerium ortin. In one embodiment, a vanadium-free catalyst for selective oxidationof nitrogen oxide with ammonia is presented having a mixed oxide supportcontaining oxides of titanium and zirconium wherein the molar ratio oftitanium oxide to zirconium oxide in the mixed oxide support is in arange of from about 70:30 to about 85:15. A first layer containing anoxide of manganese, iron, cerium or tin is present on the mixed oxidesupport in an amount of from about 0.5 mol % to about 15 mol % of themixed oxide support. A second layer containing titanium oxide is presenton the manganese, iron, cerium or tin oxide-containing first layer ofthe mixed oxide support in an amount in a range of from about 0.5 mol %to about 10 mol % of the mixed oxide support.

In another embodiment, a method of making a catalyst for selectivecatalytic reduction of nitrogen oxide is provided. The method includesthe steps of: (a) providing a mixed metal oxide support; (b) depositinga first layer containing manganese, iron, cerium or tin onto the surfaceof the mixed oxide support; and (c) depositing a second layer containingtitanium over the first layer containing manganese, iron, cerium or tin.

A method of controlling the activity or selectivity of a catalyst usedfor selective catalytic reduction of nitrogen oxides with ammonia isachieved by providing a mixed metal oxide support comprising titaniumoxide and zirconium oxide, and depositing a plurality of layersrespectively formed of a metal compound and titanium oxide onto thesurface of the mixed metal oxide. Suitable metal compounds includeoxides of manganese, iron, cerium and tin. The number of layers and thequantities of the metal compound and titanium oxide deposited arecontrolled to provide desired levels of low temperature activity or hightemperature selectivity.

In yet another embodiment, a method is provided for selective reductionof nitrogen oxides with ammonia, wherein the nitrogen oxides are presentin a gas stream. The method includes contacting the gas stream withammonia in the presence of the above-described catalyst.

Thus, utilizing (1) the technology known in the art; (2) theabove-referenced general description of the presently claimed and/ordisclosed inventive process(es), methodology(ies), apparatus(es) andcomposition(s); and (3) the detailed description of the invention thatfollows, the advantages and novelties of the presently claimed and/ordisclosed inventive process(es), methodology(ies), apparatus(es) andcomposition(s) would be readily apparent to one of ordinary skill in theart.

DETAILED DESCRIPTION OF THE INVENTION

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction, experiments, exemplary data, and/or thearrangement of the components set forth in the following description.The invention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that theterminology employed herein is for purpose of description and should notbe regarded as limiting.

There is a need for new technologies having the capability of achievingvery high reduction of NO from lean burn engines at low operatingtemperatures around 250° C. as well as high temperatures around 450° C.Certain metals such as manganese, iron, cerium and tin can be a criticalcomponent for good low temperature DeNO_(x) conversion utilizing avanadium-free catalyst. However, catalysts utilizing manganese, forexample, have been limited at higher temperature due to low ammoniaselectivity. Previously known manganese-containing catalysts also sufferthe drawback that they exhibit high selectivity for N₂O formation, whichcontributes to ozone formation and acts as a greenhouse gas.Surprisingly, it has been discovered that sequential layering ofcomponents on a vanadium-free mixed titanium and zirconium metal oxidesupport can be used to improve either high temperature selectivity orlow temperature activity.

Layering substrates is a common technique used in synthesizing othertypes of catalysts, such as three-way conversion catalysts, and forpurposes quite different from the present discovery of improving hightemperature selectivity or low temperature activity. For example, onecan subsequently deposit different materials to accomplish differentfunctions as in European Patent Application 1 053 778 A1, whichdiscloses the use of a bottom layer of a chemically inert substrate toimpart structure. Upon the bottom layer is deposited a mixed metal oxidefor high surface area and thermal stability. The catalytically activematerial is then deposited as the final layer.

Also, U.S. Pat. No. 5,863,855 discloses a layered catalyst in which thetop layer, that which is exposed most directly to the impinging fluidphase, is formulated from a material with a low abrasion resistance. Thelayer underneath contains the catalytically active phase. The purpose ofthe top layer is to capture calcium and arsenic compounds prior to theirpoisoning the active phase using attrition to remove these compoundsfrom the catalyst during normal operation before they can cause any harmto catalyst activity.

Two layers of a catalyst can also be used to prevent desorption ofunwanted compounds as described in U.S. Pat. No. 5,057,483. In thiscase, hydrogen sulfide, which is an unwanted by-product of conventionalthree-way emission catalysis that may be formed on Pt residing in thebottom layer, is destroyed by iron or nickel oxides at the top layer.

Another reason for layering a catalyst is to make use of twospecifically different morphologies to accomplish two different tasks.U.S. Pat. No. 5,597,771 describes a bottom layer where the platinumgroup metal is in intimate contact with an oxygen storage component suchas cerium oxide to promote oxidation-reduction reactions. However, it istaught that in the top layer the platinum group is specificallyprevented from intimate contact with the oxygen storage componentallowing it to catalyze the reduction of nitrogen oxides to nitrogen andthe oxidation of hydrocarbons.

A more sophisticated approach has been asserted by T. Morita, N. Suzuki,N. Satoh, K. Wada, and H. Ohno in SAE publication 2007-01-0239. In thiswork, the authors propose a top zeolite layer with Bronsted acidity.Under this layer is a platinum group metal supported on an oxygenstorage component. The purpose of the top, or zeolite layer, as statedby the authors, is to store ammonia produced during rich or reducingengine cycles. During the switch to lean cycles, the ammonia serves toconvert the NOx released by the storage component. The purpose of thebottom layer is to trap NOx during lean engine cycles and then releasethis while also converting excess NOx to ammonia during a rich cycle.

Another primary reason for layering catalysts is to prevent theinteraction of different platinum group metals where such interactioncan compromise their efficiency. For example, U.S. Pat. No. 5,989,507discloses a method where one precious metal is placed on a supportparticle of specific size while a second precious metal is placed on asupport of significantly smaller size. During synthesis, the smallerparticle diffuses closer to the substrate while the larger particle sizestays closer to the fluid phase, thus producing a concentration gradientwhere the two precious metals are intentionally segregated. This canalso be done by simply layering a top layer containing rhodium on abottom layer containing platinum as disclosed in U.S. Pat. No.4,806,519.

Thus, while catalyst layering has been used or proposed for othercatalyst systems, layering has not previously been considered forvanadium-free mixed metal oxide catalysts or for manganese, iron, ceriumor tin-containing mixed metal oxide catalysts. Nor would it beconsidered likely that layering could be used for the purpose ofimproving selectivity or increasing activity in an SCR NO_(x) removalreaction.

However, it has been discovered that improved high temperature ammoniaselectivity and reduced N₂O formation, or improved low temperatureactivity, can be achieved by providing a plurality of alternating layersrespectively formed of a metal compound and titanium oxide on thesurface of a mixed metal oxide support, wherein the metal compound is anoxide of manganese, iron, cerium, tin, or mixtures thereof, and whereinthe mixed metal oxide support contains both titanium and zirconiumoxides. Preferably, the titanium and zirconium mixed metal oxide supportis essentially free from vanadium. When referring to the mixed metaloxide support composition, the phrase “essentially free from vanadium”is used herein and in the appended claims to mean less than 0.1%vanadium or that the mixed metal oxide support contains no vanadium oronly low levels of vanadium that do not significantly contribute to thecatalytic activity of the catalysts.

The phrase “high temperature selectivity” refers herein and in theappended claims to “ammonia selectivity of a SCR DeNO_(x) catalyst attemperatures in the range of from about 400° C. to about 500° C.,” thehigh temperature range for mobile SCR DeNO_(x) applications. The phrase“low temperature activity” refers herein and in the appended claims to“NO conversion efficiency of an SCR DeNO_(x) catalyst at temperatures inthe range of from about 200° C. to about 300° C.,” the low temperaturerange for mobile SCR DeNO_(x) applications.

Preferably, the mixed oxide support consists primarily of titanium andzirconium oxides and optionally manganese oxide. The molar ratio oftitanium oxide to zirconium oxide can be any range known to thoseskilled in the art including, but not limited to, a range of from about60:40 to about 90:10. In one embodiment, the molar ratio of titaniumoxide to zirconium oxide is in a range of from about 70:30 to about85:15.

In some embodiments, the mixed oxide support has a crystalline innercore surrounded by amorphous metal oxide. The crystalline inner core canvary between anatase, rutile and a mixed oxide phase that is 2:1 Ti:Zrcalled srilankite.

The first layer of manganese oxide is present on the mixed oxide supportin an amount sufficient to achieve improved low temperature activity.Suitable amounts of manganese oxide include, but are not limited to, anamount in the range of from about 0.5 mol % to about 15 mol % of themixed oxide support, or an amount in the range of from about 1 mol % toabout 10 mol % of the mixed oxide support. The term “mol % of the mixedoxide support” (sometimes referred to herein as % equivalent) is definedas the number of mols divided by the mols of metal oxide in the mixedmetal oxide support. So, for example, if 5 mols of manganese aredeposited onto a mixed metal oxide support consisting of 80 mols of TiO₂and 20 mols of ZrO₂, the manganese oxide is present in an amount of5/(80+20) or 5 mol %.

The second layer comprises titanium oxide, the titanium oxide beingpresent on the manganese oxide first layer in an amount sufficient toachieve improved high temperature NH₃ selectivity. Such amounts include,but are not limited to, an amount in the range of from about 0.5 mol %to about 10 mol % of the mixed oxide support, and an amount in a rangeof from about 0.5 mol % to about 5 mol % of the mixed oxide support. Themol % titanium oxide is calculated as above for manganese oxide.

The second layer of titanium oxide can cause a reduction in the lowtemperature activity increase provided by the first layer of manganese.In this case, a third layer comprising manganese oxide can be depositedon the second layer of titanium oxide in amounts sufficient to improvethe low temperature activity. Such amounts include, but are not limitedto, a range of from about 0.5 mol % to about 10 mol % of the mixed oxidesupport, or a range of from about 0.5 mol % to about 5 mol % of themixed oxide support. Additional sequential alternating layers oftitanium and manganese oxide can be present in amounts necessary toachieve the desired activity and selectivity of the final catalyst.

A method of making the above described catalyst for selective catalyticreduction of nitrogen oxides is also provided. The method includes thesteps of: (a) providing a mixed metal oxide support; (b) depositing afirst layer containing manganese onto the surface of the mixed oxidesupport; and (c) depositing a second layer containing titanium over thefirst layer containing manganese.

The term “deposit” and its related forms are used herein and in theappended claims to include adsorption, ion exchange, precipitation anddeposition processes and mechanisms generally resulting in formation ofa layer or coating.

In one embodiment, the mixed metal oxide support is provided byprecipitating titanium and zirconium from an aqueous solution.Precipitation of the mixed metal oxide support is achieved by mixingsoluble titanium and zirconium compounds in water, or by mixingsolutions of dissolved titanium and dissolved zirconium, in any order.Preferably, soluble salts of titanium or zirconium are used. Nonlimitingexamples of titanium salts include titanium sulfate, titanium chloride,titanium oxychloride, and titanium nitrate. Similarly, nonlimitingexamples of zirconium salts include zirconium sulfate, zirconiumchloride and zirconium nitrate. Titanium and zirconium are precipitatedfrom solution by adjusting the pH to, for example, a pH between about 5to 10, to precipitate a mixture of titanium and zirconium oxides,hydroxides and hydrated oxides, hereinafter referred to as a mixed metaloxide precipitate. The composition of the mixed metal oxide precipitatecan be controlled by controlling the ratio of titanium to zirconiumsalts added. For example, the molar ratio of titanium to zirconium saltsadded to the aqueous solution can be within a range of from about 60:40to about 90:10. Complete precipitation results in a corresponding molarratio of TiO₂:ZrO₂ in the mixed metal oxide within the range of fromabout 60:40 to about 90:10. In one embodiment, the molar ratio ofTiO₂:ZrO₂ in the mixed metal oxide is within the range of from about70:30 to about 85:15.

In another embodiment, precipitation of the mixed metal oxide supportoccurs as described above, but includes addition of a water solublemanganese compound, or an aqueous solution of dissolved manganese, tothe combination of dissolved titanium and zirconium. Nonlimitingexamples of soluble manganese salts include manganese sulfate, manganesechloride and manganese nitrate. The titanium, zirconium and manganeseare precipitated from solution by adjusting the pH as described above.When used, the molar ratio of MnO₂:(TiO₂+ZrO₂) is typically in a rangeof from about 1:10 to 1:100.

During precipitation of the mixed metal oxide, the solution or slurry isstirred using means well known to persons of ordinary skill in the art.Unless otherwise specified or indicated by context, the terms “slurry”and “solution” are used interchangeably and include solutions,suspensions and slurries, as well as any other combination of substancesas liquid or colloidal media.

The mixed metal oxide precipitate is separated from the aqueous solutionusing any conventional technique for solid-liquid separation, such asfiltration, decanting, draining or centrifuging, and the separatedprecipitate is washed with, for example, deionized water to removesoluble ions from the precipitate. The precipitate is then dried toremove water. For the drying of this material, any temperature that iseffective for removing moisture may be used. Methods and equipment fordrying solids are well known to persons of ordinary skill in the art. Inthe laboratory, for example, the filter cake is dried in a laboratorydrying oven for about 16 hours at about 100° C.

The catalysts are prepared by sequentially layering components. A firstmanganese containing layer is deposited onto the surface of the driedmixed metal oxide support by slurrying the dried mixed metal oxideprecipitate in water and adding a soluble manganese compound. Anysoluble manganese compound can be used. Examples of suitable manganesesalts include, but are not limited to, manganese sulfate, manganeseacetate, manganese chloride, and the like. A base or pH buffer such asammonium bicarbonate is added to the slurry to insure full deposition ofthe manganese.

Preferably, one mixes the mixed metal oxide and soluble manganesecompound in order to allow for as thorough a distribution of themanganese on the mixed metal oxide support surface as possible. Methodsfor mixing are well known to persons skilled in the art.

The amount of manganese deposited onto the mixed metal oxide supportsurfaces can vary. Typically, the manganese compound is added inquantities sufficient to achieve, when deposited on the support surface,improved low temperature activity. Suitable amounts of manganese for asingle deposited coating include, but are not limited to, an amount inthe range of from about 0.5 mol % to about 15 mol % of the mixed oxidesupport. The slurry is mixed for a time sufficient to allow depositionof the manganese onto the support. Such time can vary depending onoperating conditions such as temperature, slurry concentration, etc.,but often full deposition is achieved in 30 to 60 minutes or less.

A titanium-containing layer is deposited over the manganese-containinglayer by slurrying the manganese coated mixed metal oxide with anaqueous solution containing dissolved titanium. As with the manganesecoating, one mixes the manganese-coated mixed metal oxide and solubletitanium compound(s) in order to allow for as thorough a distribution ofthe titanium as possible. Any soluble titanium compound can be used.Examples of suitable titanium salts include, but are not limited to,titanyl sulfate, titanyl chloride, titanium tetrachloride, titaniumoxylate, titanium tetraiodide, and the like. Again, a base or pH buffersuch as ammonium bicarbonate can be added to the slurry to insure fulldeposition of the titanium.

The amount of titanium deposited onto the manganese-coated mixed metaloxide support can vary. Typically the titanium compound is added in aquantity sufficient to achieve improved high temperature ammoniaselectivity, such as an amount in the range of from about 0.5 mol % toabout 10 mol % of the mixed oxide support, as described above. Theslurry is mixed for a time sufficient to allow deposition of thetitanium onto the manganese-containing layer on the support. As withdeposition of the manganese-containing layer, such time can varydepending on operating conditions such as temperature, slurryconcentration, etc., but often full deposition is achieved in 30 to 60minutes or less.

In other embodiments, additional sequentially added layers of manganesealternating with titanium are provided using the procedures describedabove. For example, and as described in more detail in Examples 1 and 2,depositing a third layer containing manganese can still further improvethe low temperature activity.

Following mixing, the slurry is filtered and dried. As discussed above,methods for filtering and drying are well known to persons of ordinaryskill in the art. For the drying of this material, any temperature thatis effective for removing moisture may be used. Preferably, greater than95% or greater than 98% of the free moisture is removed. For example,the temperature may be 100° C. or greater. After drying, the coatings onthe mixed metal oxide support comprise alternating layers respectivelyformed of manganese oxide and titanium oxide.

The above described procedures allow one to control the activity orselectivity of the catalyst by providing a mixed metal oxide supportcomprising titanium oxide and zirconium oxide, and depositing aplurality of layers respectively formed of manganese and titanium ontothe surface of the mixed metal oxide support. The number of layers andthe quantities of manganese and titanium deposited are controlled toprovide desired levels of low temperature activity or high temperatureselectivity.

The resulting catalyst, as described above or as obtained from theprocess described above, is in the form of a powder, but it can also beshaped into granules, beads, cylinders or honeycombs of variousdimensions. The catalyst can be applied to other supports that areroutinely used in the catalysis field, such as alumina or silica. Thecatalyst can also be used in catalytic systems comprising a wash coatbased on the catalyst and applied to a substrate that is, for example, ametallic or ceramic monolith.

The resulting catalyst composition may have many applications; however,these catalysts offer significant advantages for SCR DeNO_(x)applications, and are particularly suitable for the treatment of exhaustgas from automobile internal combustion engines (sometimes referred toas mobile applications). To this end, the invention also concerns theuse of a catalytic composition, as described above or as obtained by theprocesses described above, for selective reduction of nitrogen oxideswith ammonia, wherein the nitrogen oxides are present in a gas streamsuch as an automobile post combustion exhaust gas.

In order to further illustrate the present invention, the followingexamples are given. However, it is to be understood that the examplesare for illustrative purposes only and are not to be construed aslimiting the scope of the invention.

Example 1

Preparation of the mixed oxide support involves precipitation of a Ti—Zroxide. The precipitation was done by mixing titanium and zirconium saltsdissolved in water with concentrated ammonium hydroxide at a controlledpH. The salts tested in this example were titanium sulfate and chloride;and zirconium nitrate and sulfate. The pH was varied from 7 to 9 and theTiO₂:ZrO₂ molar ratios were varied from 70:30 to 85:15. Transmissionelectron micrographs suggest that the resulting particles are comprisedof a crystalline inner core surrounded by amorphous metal oxide. X-raydiffraction (XRD) results show that the crystalline core can varybetween anatase, rutile and a mixed oxide phase that is 2:1 Ti:Zr calledsrilankite. The amorphous outer layer appears to be enriched in Zr. Theresulting material was filtered and washed to remove spectator ions, asdetermined by conductivity measurements of the wash filtrate at or below1 mS/cm. The filter cake was dried for at least 16 hrs at 100° C.

Example 2

After drying the filter cake from Example 1, the solids were slurried inwater along with a soluble manganese salt, either manganese sulfate ormanganese acetate. Ammonium bicarbonate was added to the slurry toensure full deposition of Mn. The slurry was mixed for 30 min to 1 hr toallow adsorption of manganese on the support. In a second test, a 1%equivalent amount of Ti was added as titanyl sulfate after Mndeposition. A third test included the deposition of another 2%equivalent amount of Mn after the Ti deposition to further increase lowtemperature activity.

The resulting catalyst was tested in the powder form without furthershaping. A ⅜″ quartz reactor was used holding 0.1 g catalyst supportedon glass wool. The gas feed composition was 1,000 ppm NO, 1,000 ppm NH₃,5% O₂, 3% H₂O, and the balance was N₂. NO conversion was measured at250° C. and 350° C. at atmospheric pressure. The reactor effluent wasanalyzed with an infrared detector to determine NO conversion and NH₃selectivity. N₂O selectivity was determined with a feed composition of1,000 ppm NO, 1,200 ppm NH₃, 3% O₂, 2.3% H₂O, and the balance was N₂.The effluent was analyzed using a quadrupole mass spectrometer.

The results are shown in the following two tables.

TABLE 1 Comparison of catalyst selectivity and conversion N₂O NO NH₃3Selectivity Conversion Selectivity at 350 C. at 450° C. at 450° C.Catalyst Sample (%) (%) (%) 6221-175-6 6.0% Mn(II) SO4 80:20 Ti:Zr + 3.965.2  79.0 NH4HCO3 at pH 8.0 6221-176-6 6.0% Mn(II) SO4 80:20 Ti:Zr +2.1 71.5 100.0 NH4HCO3 at pH 8.0 + 1% TiO2 6221-182-6 2.0% Mn(II) SO4 on6221-176-6 + 9.8 42.9  46.3 NH4HCO3 at pH 8.0, calcined 600 C.

TABLE 2 Comparison of catalyst low temperature activity Rxn Temp 250° C.350° C. NO NH₃ NO NH₃ Catalyst Sample Conversion Selectivity ConversionSelectivity 6221-175-6 6.0% Mn(II) SO4 80:20 Ti:Zr + 22.7 100.0 68.6100.0 NH4HCO3 at pH 8.0 6221-176-6 6.0% Mn(II) SO4 80:20 Ti:Zr + 18.2100.0 59.2 100.0 NH4HCO3 at pH 8.0 + 1% TiO2 6221-182-6 2.0% Mn(II)[sulfate] on 37.7 100.0 66.5 100.0 6221-176-6 + NH4HCO3 at pH 8.0,calcined 600° C.

The original case (catalyst 6221-175-6) exhibited good conversion at450° C., but an ammonia selectivity less than 100% and a N₂O selectivityof 3.9%. Layering the equivalent 1% titania over the Mn active phaseimproved ammonia selectivity to 100% at 450° C. and reduced N₂Oselectivity to 2.1% (catalyst 6221-176-6). Conversion at lowtemperatures is reduced as shown in Table 2; however, conversion can beincreased by depositing another layer of Mn onto the catalyst asdemonstrated in Case 3 (catalyst 6221-182-6). These examples show thatlayering the various components on the catalyst allows one to controlactivity and selectivity on the final catalyst.

From the above examples and descriptions, it is clear that the presentinventive process(es), methodology(ies), apparatus(es) andcomposition(s) are well adapted to carry out the objects and to attainthe advantages mentioned herein, as well as those inherent in thepresently provided disclosure. While presently preferred embodiments ofthe invention have been described for purposes of this disclosure, itwill be understood that numerous changes may be made which will readilysuggest themselves to those skilled in the art and which areaccomplished within the spirit of the presently claimed and disclosedinventive process(es), methodology(ies), apparatus(es) andcomposition(s) described herein.

1. A composition comprising mixed metal oxide particles consistingessentially of oxides of titanium, zirconium, and optionally manganese,having on an outer surface of the particles alternating layersrespectively formed of a metal compound and titanium oxide, wherein themetal compound is selected from the group consisting of manganese oxide,iron oxide, cerium oxide, tin oxide, and mixtures thereof.
 2. Thecomposition of claim 1, wherein the mixed metal oxide particles areessentially free from vanadium.
 3. The composition of claim 2, whereinthe molar ratio of titanium oxide to zirconium oxide in the mixed metaloxide particles is in the range of from about 60:40 to about 90:10. 4.The composition of claim 2, wherein the mixed metal oxide particlescomprise a crystalline inner core surrounded by amorphous metal oxide.5. The composition of claim 4, wherein the crystalline inner corecomprises an oxide selected from the group consisting of anatase,rutile, srilankite, and mixtures thereof.
 6. The composition of claim 2,wherein a first layer of manganese oxide is present on the outer surfaceof the particles in an amount in a range of from about 0.5 mol % toabout 15 mol % of the mixed metal oxide.
 7. The composition of claim 6,wherein a first layer of manganese oxide is present on the outer surfaceof the particles in an amount in a range of from about 1 mol % to about10 mol % of the mixed metal oxide.
 8. The composition of claim 6,wherein a second layer of titanium oxide is present on the manganeseoxide first layer in an amount in a range of from about 0.5 mol % toabout 10 mol % of the mixed metal oxide.
 9. The composition of claim 8,wherein a second layer of titanium oxide is present on the manganeseoxide first layer in an amount in a range of from about 0.5 mol % toabout 5 mol % of the mixed metal oxide.
 10. The composition of claim 8,wherein a third layer of manganese oxide is present on the titaniumoxide second layer in an amount in a range of from about 0.5 mol % toabout 10 mol % of the mixed metal oxide.
 11. The composition of claim 8,wherein a third layer of manganese oxide is present on the titaniumoxide second layer in an amount in a range of from about 0.5 mol % toabout 5 mol % of the mixed metal oxide.